In many applications it is necessary to monitor the load driven by a power semiconductor switch, in order to be able to prevent damage or destruction of the power semiconductor switch. Particularly if the load is short-circuited (for example on account of a defect), the magnitude of the impedance of the driven load is very small, which can in turn lead to an overloading of the power semiconductor switch. Such an overloading can lead for example to a local overheating of the semiconductor body in which the power semiconductor switch is integrated, whereby undesirable thermal stresses arise in the semiconductor body. Thermally induced stresses in the semiconductor body can directly destroy the latter if the stress amplitude is high enough. Even when the stress amplitudes are relatively small, however, fatigue of the semiconductor material can occur, which leads to a shortening of the lifetime of the semiconductor switch.
For this reason, in known drive circuits for driving a load with a power semiconductor switch, e.g. a power metal-oxide semiconductor field-effect transistor (MOSFET), during a period of time in which the power semiconductor switch is switched off and no load current flows, a check is made at the load with the aid of a diagnostic current to ascertain whether the (ohmic) resistance of the load lies within a predetermined permissible interval. If the ohmic resistance is too low, then a short circuit is possibly present and the power semiconductor switch can be prevented from being switched on.
This method of monitoring the load consumes electrical power corresponding to the product of the diagnostic current and the resistance of the load. In many applications this power consumption in the OFF state of the power semiconductor switch is undesirable, however, since, with the power semiconductor switch switched off, the entire drive circuit for the load is intended to be put into a state of extremely low power consumption (“low power consumption mode”). However, this state of extremely low power consumption is incompatible with the power consumption during the resistance measurement by means of a diagnostic current in the OFF state of the power semiconductor switch.
In the switched-on state there is generally no requirement for restricting the power consumption. However, if the resistance measurement is carried out in the switched-on state at the load, an impermissible overheating of the power semiconductor switch may already have occurred (e.g. in the case of a short circuit) before suitable countermeasures (e.g. switching off the load current) can be implemented. Even if destruction of the power semiconductor switch is prevented by timely switching off, the local overheating can nevertheless already bring about damage resulting in a shortened lifetime of the semiconductor switch.
Consequently, there is a need for a circuit arrangement for driving a power semiconductor switch which makes it possible to monitor the connected load and which takes account of restrictions of the power consumption in the switched-off state of the power semiconductor switch. Furthermore, there is a need for a method for monitoring a load driven by a power semiconductor switch which likewise meets the abovementioned boundary conditions.